Strikes me as a completely foolish thing to do. By lifting weight with a rounded back you're loading the intervertebral discs unevenly, forcing them against the sheath in which they are encased rather than against the verterbrae. The inevitable result is a bulged or herniated disc.

Why not just train the lower back muscles in the conventional way, avoiding spinal injury? Are you really so strong in exercises like the deadlift that there's no room for improvement? What percentage of your bodyweight are you currently deadlifting?